Introduction and Learning Objectives

This tutorial will cover four protist kingdoms. They have members that are mostly photosynthetic; additionally, many members have a profound influence on humans. We will begin with a discussion of Kingdom Stramenopila, which includes the diatoms, water molds, and brown algae. Next we will examine Kingdom Rhodophyta (the red algae), the Kingdom Chlorophyta (green algae), and will conclude with slime molds, members of the Kingdom Amoebozoa. By the end of this tutorial you should have a fundamental understanding of:

The life histories and classifications of Kingdoms Stramenopila (Phyla Bacillariophyta, Chrysophyta, Oomycota, and Phaeophyta), Rhodophyta Chlorophyta, and Slime molds.

The organisms that caused the "Great Potato Famine" in Ireland, and the outbreak of downy mildew that nearly destroyed the French wine industry

The basic characteristics of algae, seaweeds, and kelps

The relationship between plants and photosynthetic protists

The relationship of the Amoebozoa to fungi and animals

Performance Objectives (for both tutorials covering the Protists):

Describe the general characteristics of the protists

Summarize the basic classification of protists

Identify monophyletic, parpaphyletic and polyphyletic groups

For each taxonomic group of protists, identify the major characteristic(s) of that group and be able to discuss representatives that demonstrate the diversity of the group

Explain different ways in which protists impact humans, either directly or indirectly

Figure 1. The Classification of Eukaryotes. (Click to enlarge)

Introduction to Kingdom Stramenopila

Stramenopiles (Fig. 3) are a remarkably diverse, monophyletic group that includes: planktonic diatoms, small single-celled or colonial freshwater protists (golden algae), large multicellular marine seaweeds (brown algae), and water molds, which include the pathogen responsible for the potato blight that drove millions of people in Ireland hungry in the nineteenth century. Although this group is diverse, they all share a common ancestral trait. Namely, the presence of hair-like projections on their flagella (Fig. 3); "stramen" means flagellum in Latin, and "pilos" means hair; hence, stramenopilans.

Kingdom Stramenopila: Phylum Bacillariophyta (Diatoms)

Figure 4. An assortment of diatoms. (Click on image to enlarge)

Diatoms are an important component of the global carbon cycle. Marine and freshwater plankton are full of photosynthetic diatoms. Some estimates suggest that they may be responsible for over 20% of annual global carbon fixation. Diatoms are encased in a glass-like silica shell, lined with perforations to allow gas exchange at the cell surface. These shells, are exceptionally beautiful structures (Fig. 4). Each one consists of two components that fit together, like the bottom and lid of a pillbox. After the death of individual diatoms, their microscopic shells sink and gradually form thick layers of sediment (diatomaceous earth). Living diatoms avoid sinking by regulating their cellular ion concentrations.

Figure 5. Diatomaceous earth.

Diatomaceous earth (Fig. 5), which essentially consists of microscopic fragments of glass from these silica shells, has numerous applications. For example, it is commonly used as a pesticide; the sharp edges of diatom shells scrape arthropod exoskeletons and gut linings, causing water loss and eventually death by desiccation. It also damages the tissues of worms and other small creatures, and often is incorporated into animal feed to reduce the occurrence of intestinal parasites. It has myriad other uses (e.g., giving scouring power to cleaning products, and allowing ultrafine filtration for scientific applications and water-purification systems).

Silica shells are fairly inert, and diatomaceous earth tends to stay where it settles. Therefore, scientists can look at the assemblages of diatom species in soil cores, where the deepest layers of the sample are the oldest and represent the most distant past. They can date the samples and examine changes in the number of warm-climate diatom species and cool-climate species over time, gaining clues about ancient climatic patterns. Actually, it is not quite this simple. Other factors, besides temperature, can influence the diatom species found, and sometimes scientists do not know the temperature preferences of extinct diatom species discovered in samples.

Diatoms are mostly photosynthetic, but there are also heterotrophic species.

Kingdom Stramenopila: Phylum Chrysophyta (Golden Algae)

Figure 6 A chrysophyte. (Click image to enlarge)

Figure 7. Dormant chrysophyte cysts. (Click image to enlarge)

Chrysophytes are planktonic, mainly freshwater organisms that possess a rich golden color due to the presence of carotenoids and xanthophylls. These different pigments allow them to expand the range of light wavelengths they can use during photosynthesis. Many golden algae are predators (as well as being photoautotrophic), using pseudopodia (cytoplasmic "feet") to prey on smaller organisms, including diatoms and bacteria. Some possess silica scales, similar in composition to the shells of diatoms; silica also coats the quiescent cysts that form under unfavorable conditions, allowing chrysophytes to remain dormant for decades (Fig. 7). Most chrysophytes are unicellular, but some species are colonial and quite elaborate in structure (e.g., Synura sp. Colony in Fig. 6) . Chrysophytes can have population explosions, similar to those in dinoflagellates (HABs – Harmful Algal Blooms), that cause a red-tide like occurance. These “golden tides” have result in large fish kills in some freshwater environments (Fig. 8).

A Real Life Example: Diatoms and symbioses

Viruses are small protein-enclosed particles that contain their own genetic material (either DNA or RNA) but can’t reproduce outside of a host cell. They are obligate parasites that need to infect host cells to replicate and they infect a narrow range of hosts. For example, the viruses that cause most common colds in humans (rhinoviruses) are only able to infect cells in the upper respiratory tract of humans. After infection, the host cells die when new viruses burst from them.

Recently, biologists have come to realize that viruses play important roles in regulating and modifying most aspects of important biological processes. For example, we now know that viruses are incredibly abundant in the oceans. Measurements suggest that a milliliter of sea water contains ~107 viruses and these viruses infect every type of organism in the sea from bacteria to whales.

Recently, a virus has been discovered (Chaetoceros salsugineum nuclear inclusion virus (CsNIV)) that infects Chaetoceros salsugineum, a species of diatom. Diatoms of the genus Chaetoceros are abundant components of marine plankton communities. They are known to contribute to global atmospheric oxygen and carbon dioxide levels and are a major source of nutrition for organisms higher on the food chain. Several species of Chaetoceros are known to parasitize fish by clogging their gills.

How do Chaetoceros contribute to global atmospheric oxygen and carbon dioxide levels?

What effect could a decline in viruses that infect Chaetoceros have on populations of fish that are parasitized by Chaetoceros?

Kingdom Stramenopila: Phylum Oomycota

Oomycetes (Fig. 9) include the water molds and downy mildews. Some are unicellular, while others are colonial. Most are heterotrophic decomposers that feed on dead and decaying organic matter in aquatic and terrestrial environments, but some attack living plants and animals. The name "oomycete" means "egg fungi," which is a reference to the reproductive structures of sexually reproducing oomycetes.

Figure 10. Oomycete hyphae. (Click to enlarge)

Although oomycetes are not closely related to fungi, the two groups have some similarities. In particular, both groups are heterotrophs that break down food externally and then absorb nutrients from their surroundings. Additionally, many of the multicellular oomycetes form hyphae, which are very similar to fungal hyphae although the structures are not identical (Fig. 10).

However, there are more differences than similarities. For example, the cell walls of oomycetes are composed of cellulose, not chitin (the compound found in the cell walls of fungi). Unlike true fungi, which are haploid in the feeding stage (and the majority of their life cycle), the hyphae of oomycetes are composed of diploid cells.

The resemblance between fungi and oomycetes is an example of convergent evolution, the process by which unrelated organisms that occupy similar environments evolve similar functional traits. For example, oomycetes and fungi are decomposers and therefore stand to gain an advantage from maximizing their surface area for the absorption of food; thus, the filamentous growth form would have been favored by selection and therefore evolved independently in both lineages.

Oomycetes Impact on Humans

Oomycetes have been responsible for a number of catastrophic historic events, including the outbreak of downy mildew that nearly wiped out the French wine industry in the late nineteenth century. Plasmopara (Figs.11 and 12), a native of the New World, was inadvertently brought to France from the United States in 1870 in a shipment of American grape root stock. It quickly became a devastating problem. A mixture of lime and copper sulfate, the first chemicals used to combat a plant pathogen, successfully treated grapes affected by this organism.

Figure 13 (Click on image to enlarge)

Figure 14. (Click on image to enlarge)

Figure 15. (Click on image to enlarge)

The oomycete Phytophthora infestans, or potato blight, was responsible for another cataclysmic event, the Great Potato Famine, which killed nearly one-million people in Ireland in the late 1800s, and drove one-and-a-half-million more out of the country. Phytophthora (Fig. 13) secretes enzymes that break down leaf and stem tissue (Fig. 14, killing plants very rapidly. Tubers can become infested, turning soft and black, seemingly overnight. Potato fields in Ireland became infested with Phytophthora, which thrived in the cool damp climate of Ireland and wiped out nearly the entire country's crop in one week. In nineteenth-century Ireland, potatoes were the primary food source of the poorest classes, who often grew nothing other than potatoes. The loss of this staple crop led to mass starvation (Fig. 15). Visit this link (http://www.youtube.com/watch?v=D4_TiXrd1Mg) and be able to describe the crops that have been infected by this oomycete in Pennsyvania. How does it affect these crops? What conditions led to the outbreak in 2009? How can you keep this disease from spreading?

Kingdom Stramenopila: Phylum Phaeophyta (Brown Algae)

Figure 16 (Click image to enlarge)

Phaeophytes, or brown algae (Fig. 16), include the largest of the protists, with some growing over 100 feet in length. The giant multicellular species that comprise "kelp forests" in temperate marine waters belong to this group. The edible alga "kombu," which is harvested by the Japanese, is also a phaeophyte. It is particularly rich in minerals, as are other marine algae. Like the chrysophytes, phaeophytes contain specific accessory pigments that give them their characteristic colors.

Figure 17. (Click image to enlarge)

Along with the two kingdoms that we will discuss next, Rhodophyta and Chlorophyta, Phylum Phaeophyta includes a number of seaweeds. Although it is not a taxonomic term, the word "seaweed" is a useful way of distinguishing large intertidal algae from other species (e.g., planktonic algae). Many types of seaweed, including those within the Phylum Phaeophyta, have complex structures that are reminiscent of plants. The thallus refers to the entire body of any seaweed that is plant-like in appearance. The thallus consists of three main parts: a stipe, which is analogous to the stem of plants; a holdfast, which secures the seaweed to a substrate; and leaf-like blades, which provide extensive surface area for photosynthesis, much as leaves do for plants.

Figure 18 (Click on image to enlarge)

Another word used informally to describe a specific type of alga is "kelp," which refers to giant seaweeds that grow in the deeper waters outside of the intertidal zone. All kelps, which form vast "forests" that support thriving marine ecosystems, are phaeophytes. As one can see(Fig. 18), these organisms can reach very impressive sizes. Visit this web site to take a tour of a kelp forest ( http://www.youtube.com/watch?v=GcbU4bfkDA4). What types of animals live in a kelp forest ecosystem? Why is it important for their reproduction?

Kingdom Rhodophyta

Members of Kingdom Rhodophyta differ from other eukaryotic algae because they do not have flagellated cells at any point in their life cycles. DNA sequence data (and other sources) indicate that this lineage arose independently, sometime before Kingdom Stramenopila.

Most rhodophytes are marine algae (Fig 19), but a number of species live in freshwater and some even inhabit terrestrial niches (Fig 20). Like many of the photosynthetic eukaryotes, rhodophytes are characterized by accessory pigments in their chloroplasts, which endow them with unique colors. In the case of rhodophytes, also called red algae, pigments called phycobilins produce rich shades of pink, scarlet, and red that are so deep that they approach black. These red pigments, in particular the phycobilin phycoerythrin, allow rhodophytes to photosynthesize at water depths that only high-energy blue and green light can penetrate. In fact, the color of rhodophyte species tends to be correlated with the depth where they commonly occur; deep-water species are often nearly black, concentrated with phycoerythrin, whereas shallow-water species can contain so few accessory pigments that they appear almost green, having few pigments to mask the green color of chlorophyll.

Many red algae are large multicellular organisms, although they are not in the same size class as the largest of the brown algae. A number of these species are of economic importance; the odds are very good that you've eaten red algae or that you have relied on its derivatives. The seaweed used in sushi (nori) is a red alga.

Kingdom Chlorophyta

Figure 21 (Click image to enlarge)

Chlorophytes, or green algae (Fig. 21), are more closely related to plants than are the other photosynthetic protists. The chlorophylls found in green algae (chlorophyll a and chlorophyll b) is similar (can’t we just say it is the same) to that of plants. Green algae also store fixed carbon as starch and they have cell walls composed of cellulose. Because they do not have many of the adaptations to a terrestrial environment that plants share, most systematists continue to classify green algae as a separate kingdom, but recognize that land plants arose from within this group.

There is a tremendous amount of diversity within this kingdom. Chlorophytes occupy a remarkable array of habitats, including marine, freshwater and terrestrial environments. A number are seaweeds, most are freshwater algae, and some terrestrial chlorophytes (lichens) live in a symbiotic association with fungi. The 7,000 species that comprise this kingdom range from unicellular to colonial to truly multicellular. Separate evolutionary events might have led to the evolution from single-celled green algae to colonial organisms (e.g., the beautiful Volvox), to multinucleate single-celled seaweeds, and to multicellular seaweeds (e.g., Ulva). The photos below (Fig. 22) show the diversity of this group.

Figure 22. The diversity of green algae.

Kingdom Chlorophyta also exhibits many forms of reproduction; most species reproduce asexually and sexually. For example, Spirogyra is composed of long filaments of cells that can break apart and divide to produce new individuals. Alternatively, bridges can form between different individuals, allowing the exchange of gametes. Like the brown and red algae, many green algae also display alternation of generations during their sexual cycles, switching between haploid and diploid forms; we will cover this when we discuss plant reproduction.

Slime Molds

Figure 23. A myxomycete, or plasmodial slime mold. (Click to enlarge)

Figure 24. A plasmodial slime mold, growing on a conifer. (Click to enlarge)

At one time, Slime molds were classified with the fungi because of their heterotrophic feeding habits and appearance. Further study of these organisms has shown that they are members of the Kingdom Amoebozoa, which includes the familiar protist Amoeba. Therefore, these similarities likely arose independently in this group and hence the similarities between fungi and slime molds are an example of convergent evolution, as we saw with oomycetes and fungi. Myxomycetes, or plasmodial slime molds, are among the most bizarre and beautiful organisms on Earth, often brilliantly colored in shades of yellow, orange, or pink (Figs. 23 and 24). They can be very large and have given some gardeners a scare when found growing on a damp pile of bark mulch (Fig. 25). Despite their size, plasmodial slime molds are unicellular; they are large bags of cytoplasm. However, within each "cell" are many nuclei. The synchronous division of these nuclei has made slime molds very useful research organisms for the study of mitosis; additionally, many scientists study cytoplasmic streaming (the circular flow of cytoplasm that aids in moving materials within the cell) in myxomycetes because it is a relatively easy phenomenon to observe due to the size of these organisms.

Acrasiomycetes, or cellular slime molds, are also very unusual organisms. They spend most of their lives as amoeba-like single cells, but when resources are scarce they converge, joining with other cells to form units that have coordinated functions, as seen in multicellular organisms (Fig. 26). Unlike plasmodial slime molds, the cells within the acrasiomycetes retain their cell membranes and their ability to live independently. Cellular slime molds have been used as a model system for scientists studying the behaviors of individual cells and coordination among groups of cells. Researchers studying these organisms have an active online community, Dictybase, providing a resource for the hundreds of scientists working on the cellular slime mold Dictyostelium. You can watch videos of their life cycle at http://dictybase.org/Multimedia/index.html; the phagocytosis video shows how they eat using amoeboid motion and pseudopodia to engulf their food. What component of the cytoskeleton is responsible for this motion?

Summary

Protists are ecologically and economically significant organisms; they are responsible for a large portion of the Earth's fixed carbon and for a number of plant and animal diseases, some of which have caused considerable devastation to humans and their crops. Considering their small size, their diversity is astounding. The beginnings of multicellularity in early protists paved the way for even greater structural complexity and diversity in taxa such as plants, animals, and fungi. For more information on the systematics and taxonomy of protists (and all other life forms), the web site of the University of California-Berkeley's Museum of Paleontology is an excellent resource, although their system of classification differs somewhat from the system used in this course.

We also introduced the slime molds.. In a hike through the woods, you might encounter a brightly colored, fungus-like structure spreading over the surface of a decaying tree trunk; most likely, you are observing a slime mold. These are members of the Amoebozoa, the protist kingdom most closely related to fungi and animals. We wil explore the origins of those groups in Tutorials 16 and 18.

Terms

After reading this tutorial, you should have a working knowledge of the following terms: